During the p-n junction formation, when an electron diffuses from n → p, it leaves behind an:

ionised donor on n-side

The correct answer is Option (2) → ionised donor on n-side

The region near the junction becomes depleted of free charge carries (electrons and holes). This region is called the depletion region.

→ Ionised donors on the n-side.

→ Ionised acceptor on the p-side.